Method of forming blade roots



Dec. 1, 1953 J. ATKINSON 2,660,779

METHOD OF FORMING BLADE ROOTS Filed Jan. 13, 1949 2 Sheets-Sheet l FIG!FIG.2

a H gave/1101' mt fzr ncys Dec. 1, 1953 J. ATKINSON METHOD OF FORMINGBLADE ROOTS 2 Sheets- Sheet 2 Filed Jan. 15, 1949 I I 1 zgenfor Aflor'neyJ Patented Dec. 1, 1953 UNITED STATES PATENT OFFICE it 2,660,779I M OD F 0 v Gl LADE RO JosephAtkinson, London, England ApplicationJanuary 13,1949, Serial No. 70,723

Claims priority, application Great Britain January 15, 1948 Thisinvention relates to an improved method for forming the root or anchorportion of the blades used in bladed fluid-flow machines such asturbines and compressors, and relates particularly to blades of thetype'which areformed with a profiled root adapted to be secured in asocket (hereinafter referred to as blades of the type described).

, The invention is mainly of interest in connection with the productionof blades which are subjected in use to high temperatures and stresses,and which therefore require to be made of specially developed steels oralloys, such as nickel-chrome alloys, which may be diilicult andexpensive to machine. E'xamplesof such blades are the turbine rotorblades of gas turbine ,machines, and in a lesser degree the compressorrotor blades (in cases where bladed compressors of the axial flow typeare used).

One known method of securing the blades to the turbine rotor disc of agas turbine is to provide the periphery of the rotor disc with serratedslots extending in a generally radial direction, complementaryserrations being formed on the blade roots so that they can be pressedinto the slots and secured, for instance by peeping. Where the serratedblade root, and the corresponding slot, are tapered, such a method ofblade mounting is generally known in the art (from the profile of theblade root) as the firtree mounting, and it will be hereinafterdesignated by this term. At present fir-tree mountings are in wide use.

Fir-tree roots have hitherto been usually made by a machining operation,such as broaching, and one object of the present invention is to providean alternative method of making such roots which eliminates or reducesmachining operations, thus cheapening production, and which, at least inthe case of some materials, also has a beneficial effect on the materialtreated.

Broadly the invention provides, for blades of the type described, themethod of forming the root contour wholly or partly by means ofpressure-working. I

By pressure-working is meant a rolling, swaging, stamping, coining orsimilar operation.

The pressure working may most advantageously comprise a rollingoperation, which will usually be cold, but may be hot if the nature ofthe material which is being worked requires it. The rolls may be of anysuitable material, such as hardened steel or tungsten carbide.

Cold pressure working provides an improved surface finish for most ofthe metallic materials I 3 claims. .(01. se -156.8)

' likely tobe useful for the turbine blades of;gas

turbine machines, and may have abeneficial eiiect in other respects; forexample, alloys of the 18 chromium 8 nickel type can'have their creeplimit raised by cold work (see Zschokke,

BrownBoveri Review 1946, v. 33, pp. 227-233). Other highly alloyedsteels are also improved in the same respect by cold work (seeCornelius, Metallwirtschaft, 1939, v. .18 pp. 399-403, and pp. 419-421).The degree of such improvement does however, depend on .the proximityoithe operating temperature of the blade root to the re-crystallisationtemperature. With the general run of turbine rotor disc rim temperaturesthe diil'erence between the operating temperature and there-crystallisation temperature is such that substantial improvementresults.

Some alloys, intended to operate at lower temperatures, such as aluminumalloys, bronzesand stainless steels, for compressor blades, lose aportion of their highly creep-resisting properties when cold worked.With such alloys, or where the operating temperature is near tore-crystallisation temperature, it is preferable to subject at least theroot of the blade (after pressure workmachine in which the work is heldstationary and the rolls are moved linearly over the .work. Fig. .1 is afront elevation of the movable ram carrying the rolls, Fig. 2 is atransverse section through the ram, showing the stationary table inwhich the blades are fixed, and Fig. 3is a side elevation of the ram. 7

Fig. 4 is a side e1evati0n, and Fig. 5 is a fragmentary plan, of analternative rolling machine in which the blades are fed linearly throughthe rolls.

Figs. 6 and '7 are detail views ofmo'dified types of roll.

Referring to Figs. 1-3, the machine there shown comprises in essentialsa vertically movable ram I, in which three slides 2 are mounted forhorizontal sliding movement. Each slide is resiliently biassed to acentral position by a spring-pressed ball or plunger device 3, and theslide can move against such spring pressure in

